SITL: [minor] remove an unused string, mark it unused in ctor

Author: hunt0r

libraries/SITL/SIM_Aircraft.cpp

@@ -50,8 +50,7 @@ Aircraft *Aircraft::instances[MAX_SIM_INSTANCES];
   parent class for all simulator types
  */
 
-Aircraft::Aircraft(const char *frame_str) :
-    frame(frame_str)
+Aircraft::Aircraft(const char *unused_frame_str)
 {
     // make the SIM_* variables available to simulator backends
     sitl = AP::sitl();

libraries/SITL/SIM_Aircraft.h

@@ -53,7 +53,7 @@ namespace SITL {
  */
 class Aircraft {
 public:
-    Aircraft(const char *frame_str);
+    Aircraft(const char *unused_frame_str);
 
     // called directly after constructor:
     virtual void set_start_location(const Location &start_loc, const float start_yaw);
@@ -276,7 +276,6 @@ class Aircraft {
     uint32_t last_frame_count;
     uint8_t instance;
     const char *autotest_dir;
-    const char *frame;
     bool use_time_sync = true;
     float last_speedup = -1.0f;
     const char *config_ = "";